With the recent dramatic increase in the costs of all forms of energy and the attendant efforts to procure and utilize older, more traditional energy resources as well as petroleum resources, the need for large quantities of inert gases either as moderators or pressure sustaining mediums has become of strategic importance. Carbon dioxide gas has been one of the inert gas mediums to be utilized in such projects. Carbon dioxide has been a readily achieveable combustion product which although having relatively low production expense is still costly enough that a cyclic use of the gas would benefit the economics of any of the number of energy related processes to which this invention is related.
Oil shale retorting and specifically in-situ oil shale retorting is one of the processes to which the present invention concerning the recycle of carbon dioxide is particularly pertinent. Oil shale is a sedimentary geologic formation generally found in greatest abundance in the western states of the United States. Various oil shales contain varying percentages of a hydrocarbonaceous component which is called kerogen. Kerogen comprises the petroleum resource which has made oil shale of significant interest to those attempting to meet the energy requirements of the industrialized world. By mining oil shale and heating it to the liquefaction point of the kerogen component, the hydrocarbonaceous component of the oil shale can be readily recovered for further refinement into oil products which are similar to the liquid petroleum oil products presently recovered from oil wells. In order to liquefy the kerogen component of oil shale it is necessary to retort the oil shale either in-situ within the ground or within a surface retort being supplied with mined oil shale ore. In an in-situ oil shale retort process, the oil shale is blasted into a concise rubble pattern in which the outer limits of the blast zone constitute the retort container or vessel. In surface retorting, traditionally mined oil shale is supplied in a particulate form to a traditional retort vessel where the necessary heat is supplied to liquefy and separate the kerogen content. The two forms of oil shale retorting have different modes of operation, both of which are well known and documented in the prior art. Generally, in-situ oil shale retorting is a batch opertion in which the oil shale is ignited by a burning gas and then the combustion is continued downwardly through the rubblized oil shale formation by the combustion of char contained in the oil shale until the in-situ blast pattern is exhausted. This is a non-steady state operation. In comparison, surface retorting of oil shale is usually done in a continuous manner under steady state conditions in which the burn zone is maintained centrally within the retort and the material to be processed, namely oil shale, is fed downwardly through the retort vessel for consumption. In either of these combustion processes a steady flow of an oxidant gas is required. Only an initial flow of combustible fuel gas is required in order to initiate the burn. The burn is sustained by burning the char residue of the particulate shale. Using either oxygen gas or air as the typical oxidant requires an additional gas as a diluent in order to moderate the peak combustion temperatures to avoid melting the shale to a slag and to avoid producing excessive energy consuming carbonate decomposition. Steam and carbon dioxide gas are known retort diluents. During the operation of the retort, substantial quantities of liquid hydrocarbon oil and off-gases are produced. The off-gases comprise combustion products, oil shale volatiles and diluent gas. The liquid phase of the effluent from the retort is separated from the gaseous phase of the effluent and the gaseous phases can then be cleaned and vented or recycled alternately. Of course, the liquid phase is refined as the primary fuel source. Various sulfur compounds such as hydrogen sulfide and carbonyl sulfide are found in the off-gases from the retort and these gases present a problem to the disposal or use of the retort off-gases. It has been found that the sulfides can be absorbed on the spent oil shale if the acid gas portion of the off-gas is recycled to the retort. Additionally, when the off-gas is depleted of any BTU fuel components the off-gas serves as an excellent moderator or diluent gas which can be combined with the retort influent. In this manner, the diluent gas is mixed with an oxidant such as oxygen gas and the mixture is fed to the retort to sustain the char burn.
In surface and in-situ combustion type coal gasification processes, moderators are typically added to the input air or oxygen. In surface gasifier retorts, steam has typically been used to hold peak temperatures to levels where the coal ash will not slag. In in-situ coal gasification processes, steam has been added to avoid excessive temperatures with high heat losses into surrounding strata and to avoid burnout of the oxidant injection lance. Steam has the advantage that it is easily separated as condensate by cooling the gasifier effluent. It has the disadvantage that the condensate requires expensive treatment to remove contaminants and that energy requirements for steam generation are high. In the established Lurgi dry ash moving bed gasifier retort using steam and oxygen, the energy required for the steam is 3 to 4 times greater than that required to supply the oxygen. Carbon dioxide has been proposed as a combustion moderator for coal gasification, but has not been widely used even though it has been potentially available for recycle from the gasifier effluent. High energy requirements of existing processes for separating the CO.sub.2 for recycle have presumably discouraged its use.
In oxygen fireflooding, an oxidant gas is used to combust an oil formation which does not naturally produce due to the lack of natural in-situ pressure, high oil viscosity or unfavorable formation structure. The oxidant gas is supplied in an injection well to spontaneously combust the formation or to sustain artificially initiated combustion of the oil. The combustion heats the oil, lowers its viscosity and allows the oil to be recovered from a producing well. A significant amount of CO.sub.2 as well as other gases from the combustion are recovered with the produced oil. These gases can be separated in the present invention and with additional sulfur treatment, the CO.sub.2 gas can be pipelined to other enhanced recovery operations.
In carbon dioxide miscible flood enhanced oil recovery operations, high pressure carbon dioxide is injected into a partially depleted oil reservoir. The carbon dioxide serves to extract and displace the residual oil to a production well that discharges carbon dioxide and recovered oil to the surface at reduced pressure. The oil product liquid phase is separated from the carbon dioxide and hydrocarbon gas phase. The gas can be processed in the present invention process to separate a hydrocarbon gas product and carbon dioxide for reinjection to the reservoir.
In the above-stated combustion processes in which carbon dioxide recycling occurs, it is environmentally as well as economically beneficial to recycle the carbon dioxide off-gases as a diluent gas for the combustion operation and to absorb any sulfur containing components from the off-gases onto the remaining combusted media, such as spent oil shale or coal ash. Such a method avoids the costly preparation of steam diluent and provides greater selectivity of diluent proportion that a more conventional air mixture diluent. Of course, the recycle of off-gases offers an attractive method by which to avoid contamination of the environment with noxious sulfur contaminants such as sulfides in various forms.
Various prior art processes have been developed for the recycling of such off-gases as are the by-product of coal gasification, oil shale retorting, oxygen fireflooding and other enhanced oil recovery operations. These prior art processes generally suffer from high energy consumption and a complexity of process apparatus which requires high capital expenditure.
In U.S. Pat. No. 2,886,405, a process is disclosed for the separation of carbon dioxide and hydrogen sulfide from gas mixtures utilizing a chemical absorbent solvent, such as hot potassium carbonate. As is typical in chemical solvents, the enriched solvent is regenerated by a boiling and steam stripping operation. Such regeneration is an energy intensive operation.
The prior art in U.S. Pat. No. 4,014,575 teaches that off-gases from oil shale retorting can be recycled through spent oil shale beds for the deposition of sulfur compounds from the off-gas onto the particles of the oil shale bed. This can be done in conjunction with the water scrubbing of the off-gases in a Venturi scrubber.
Another method has been utilized to scrub the off-gases from oil shale retorting wherein water containing basic components from an oil shale retort bed is contacted with the acid gas containing off-gas stream of an operating oil shale retort. The basic pH water neutralizes the acid off-gases and the later can be recycled for retorting or burned if sufficient BTU energy can be derived. This process is described in U.S. Pat. No. 4,117,886.
In U.S. Pat. No. 4,158,467, a process for the recycling of oil shale retort off-gases is disclosed wherein the hot potassium carbonate solvent of U.S. Pat. No. 2,886,405, mentioned above, is utilized. As stated before, the utilization of chemical absorbent solvents in such an operation is energy intensive due to the complexity of regenerating such solvents for reuse. Additionally, the chemical absorption process is essentially nonselective. That is complete absorption of acidic sulfur compounds would be accompanied by complete absorption of contained carbon dioxide.
The removal of acid gas components from gas streams is discussed in U.S. Pat. No. 4,169,133 wherein the carbon dioxide acid gas component is frozen out of the main gas stream. A process wherein a solid product is produced from a gas clean-up operation is not conducive to the recycling of such a component, such as in the present invention.
In U.S. Pat. No. 4,169,506, the scrubbing of off-gases from in-situ retorting of oil shale is set forth. The scrubbing utilizes caustic soda in conjunction with a deoiling process. In this instance, the scrubbed sulfur components are passed to a Claus plant for refinement to elemental sulfur.
In South African published Application 77/7157 of Dec. 1, 1977 a process is disclosed for the separate removal of sulfides and carbon dioxide from a coal gasification gas stream. Externally supplied refrigeration is necessary to operate a complex solid/liquid absorbent stream in a process which operates on carbon dioxide containing streams in the 55% carbon dioxide range. Corresponding U.S. Pat. No. 4,270,937 of June 2, 1981 discloses similar subject matter.
Attempts by the prior art to solve the various problems in the economical provision of a diluent or pressurizing gas for a oxidant fed combustion process or a pressure enhanced oil recovery operation have been deficient for several reasons, including: the energy intensive nature of the combustion scrubbing recovery operations, the necessity for regeneration of chemical solvents by steam stripping operations, the need for large quantities of water for scrubbing operations in retorting or combustion locations which may be deficient in adequate water resources to make such recovery systems operational and the uneconomical separation and recycle of pressurizing fluids in enhanced oil recovery operations. Furthermore, many of the prior art systems fail to incorporate a system for the separation of hydrocarbons from the carbon dioxide diluent and sulfide components of the off-gas streams related to the present invention. The hydrocarbons contemplated are specifically the C.sub.4 + hydrocarbons which are produced in these fuel conversion combustion processes and necessarily from the intermingling of a pressurizing gas with a petroleum reservoir.
The present invention overcomes these drawbacks by providing a low energy, low temperature system for the recovery of recyclable gases including propane from the off-gases of carbonaceous combustion retorting and pressure enhanced oil recovery operations. The present invention achieves this recovery of recyclable gases such as carbon dioxide, acidic sulfide gases and propane, by low temperature separation in two stages. Furthermore, the present invention process does not require the utilization of potentially scarce and valuable water resources at the site of the recycling operation.
With respect to oxygen fireflooding and other enhanced oil recovery operations, such as CO.sub.2 miscible flood enhanced recovery, the present invention can be used to extract and pipeline bulk CO.sub.2 after additional sulfur removal in the former process or to recycle CO.sub.2 in a non-combustion process in the latter operation.